The aim of this research was to analyze the effects of increased N or P
availability, increased air temperature, and decreased light intensity on wet
sedge tundra in northern Alaska. Nutrient availability was increased for 6–9
growing seasons, using N and P fertilizers in factorial experiments at three
separate field sites. Air temperature was increased for six growing seasons,
using plastic greenhouses at two sites, both with and without N + P fertilizer.
Light intensity (photosynthetically active photon flux) was reduced by 50% for
six growing seasons at the same two sites, using optically neutral shade cloth.
Responses of wet sedge tundra to these treatments were documented as changes in
vegetation biomass, N mass, and P mass, changes in whole-system CO2 fluxes, and
changes in species composition and leaf-level photosynthesis.

Biomass, N mass, and P mass accumulation were all strongly P limited, and
biomass and N mass accumulation also responded significantly to N addition with
a small N × P interaction. Greenhouse warming alone had no significant effect
on biomass, N mass, or P mass, although there was a consistent trend toward
increased mass in the greenhouse treatments. There was a significant negative
interaction between the greenhouse treatment and the N + P fertilizer treatment,
i.e., the effect of the two treatments combined was to reduce biomass and N mass
significantly below that of the fertilizer treatment only. Six years of shading
had no significant effect on biomass, N mass, or P mass.

Ecosystem CO2 fluxes included net ecosystem production (NEP; net CO2 flux),
ecosystem respiration (RE, including both plant and soil respiration), and gross
ecosystem production (GEP; gross ecosystem photosynthesis). All three fluxes
responded to the fertilizer treatments in a pattern similar to the responses of
biomass, N mass, and P mass, i.e., with a strong P response and a small, but
significant, N response and N × P interaction. The greenhouse treatment also
increased all three fluxes, but the greenhouse plus N + P treatment caused a
significant decrease in NEP because RE increased more than GEP in this
treatment. The shade treatment increased both GEP and RE, but had no effect on
NEP. Most of the changes in CO2 fluxes per unit area of ground were due to
changes in plant biomass, although there were additional, smaller treatment
effects on CO2 fluxes per unit biomass, per unit N mass, and per unit P mass.

The vegetation was composed mainly of rhizomatous sedges and rushes, but
changes in species composition may have contributed to the changes in vegetation
nutrient content and ecosystem-level CO2 fluxes. Carex cordorrhiza, the species
with the highest nutrient concentrations in its tissues in control plots, was
also the species with the greatest increase in abundance in the fertilized
plots. In comparison with Eriophorum angustifolium, another species that was
abundant in control plots, C. cordorrhiza had higher photosynthetic rates per
unit leaf mass. Leaf photosynthesis and respiration of C. cordorrhiza also
increased with fertilizer treatment, whereas they decreased or remained constant
in E. angustifolium.

The responses of these wet sedge tundras were similar to those of a nearby
moist tussock tundra site that received an identical series of experiments. The
main difference was the dominant P limitation in wet sedge tundra vs. N
limitation in moist tussock tundra. Both tundras were relatively unresponsive to
the increased air temperatures in the greenhouses but showed a strong negative
interaction between the greenhouse and fertilizer treatments. New data from this
study suggest that the negative interaction may be driven by a large increase in
respiration in warmed fertilized plots, perhaps in relation to large increases
in P concentration.